David Rientjes <rientjes@xxxxxxxxxx> writes:
> On Wed, 11 Feb 2015, Vitaly Kuznetsov wrote:
>
>> If newly added memory is brought online with e.g. udev rule:
>> SUBSYSTEM=="memory", ACTION=="add", ATTR{state}="online"
>> the following deadlock is observed (and easily reproducable):
>>
>> First participant, worker thread doing add_memory():
>>
>> [ 724.948846] kworker/0:1 D ffff88000412f9c8 13248 27 2 0x00000000
>> [ 724.973543] Workqueue: events hot_add_req [hv_balloon]
>> [ 724.991736] ffff88000412f9c8 0000000000000000 ffff88003fa1dc30 00000000000151c0
>> [ 725.019725] 0000000000000246 ffff88000412ffd8 00000000000151c0 ffff88003a77a4e0
>> [ 725.046486] ffff88003fa1dc30 00000001032a6000 ffff88003a7ca838 ffff88003a7ca898
>> [ 725.072969] Call Trace:
>> [ 725.082690] [<ffffffff81aac0a9>] schedule_preempt_disabled+0x29/0x70
>> [ 725.103799] [<ffffffff81aae33b>] mutex_lock_nested+0x14b/0x470
>> [ 725.122367] [<ffffffff815ed773>] ? device_attach+0x23/0xb0
>> [ 725.140992] [<ffffffff815ed773>] device_attach+0x23/0xb0
>> [ 725.159131] [<ffffffff815ecba0>] bus_probe_device+0xb0/0xe0
>> [ 725.177055] [<ffffffff815ea693>] device_add+0x443/0x650
>> [ 725.195558] [<ffffffff815ea8be>] device_register+0x1e/0x30
>> [ 725.213133] [<ffffffff81601790>] init_memory_block+0xd0/0xf0
>> [ 725.231533] [<ffffffff816018f1>] register_new_memory+0xb1/0xd0
>> [ 725.250769] [<ffffffff81a961cf>] __add_pages+0x13f/0x250
>> [ 725.269642] [<ffffffff81063770>] ? arch_add_memory+0x70/0xf0
>> [ 725.288764] [<ffffffff81063770>] arch_add_memory+0x70/0xf0
>> [ 725.306117] [<ffffffff81a95f8f>] add_memory+0xef/0x1f0
>> [ 725.322466] [<ffffffffa00293af>] hot_add_req+0x33f/0xf90 [hv_balloon]
>> [ 725.342777] [<ffffffff8109509f>] process_one_work+0x1df/0x4e0
>> [ 725.361459] [<ffffffff8109502d>] ? process_one_work+0x16d/0x4e0
>> [ 725.380390] [<ffffffff810954bb>] worker_thread+0x11b/0x450
>> [ 725.397684] [<ffffffff810953a0>] ? process_one_work+0x4e0/0x4e0
>> [ 725.416533] [<ffffffff8109ac33>] kthread+0xf3/0x110
>> [ 725.433372] [<ffffffff8109ab40>] ? kthread_create_on_node+0x240/0x240
>> [ 725.453749] [<ffffffff81ab1dfc>] ret_from_fork+0x7c/0xb0
>> [ 725.470994] [<ffffffff8109ab40>] ? kthread_create_on_node+0x240/0x240
>> [ 725.491469] 6 locks held by kworker/0:1/27:
>> [ 725.505037] #0: ("events"){......}, at: [<ffffffff8109502d>] process_one_work+0x16d/0x4e0
>> [ 725.533370] #1: ((&dm_device.ha_wrk.wrk)){......}, at: [<ffffffff8109502d>] process_one_work+0x16d/0x4e0
>> [ 725.565580] #2: (mem_hotplug.lock){......}, at: [<ffffffff811e6525>] mem_hotplug_begin+0x5/0x80
>> [ 725.594369] #3: (mem_hotplug.lock#2){......}, at: [<ffffffff811e656f>] mem_hotplug_begin+0x4f/0x80
>> [ 725.628554] #4: (mem_sysfs_mutex){......}, at: [<ffffffff81601873>] register_new_memory+0x33/0xd0
>> [ 725.658519] #5: (&dev->mutex){......}, at: [<ffffffff815ed773>] device_attach+0x23/0xb0
>>
>> Second participant, udev:
>>
>> [ 725.750889] systemd-udevd D ffff88003b94fc68 14016 888 530 0x00000004
>> [ 725.773767] ffff88003b94fc68 0000000000000000 ffff8800034949c0 00000000000151c0
>> [ 725.798332] ffffffff8210d980 ffff88003b94ffd8 00000000000151c0 ffff880037a69270
>> [ 725.822841] ffff8800034949c0 0000000100000001 ffff8800034949c0 ffffffff81ff2b48
>> [ 725.849184] Call Trace:
>> [ 725.858987] [<ffffffff81aac0a9>] schedule_preempt_disabled+0x29/0x70
>> [ 725.879231] [<ffffffff81aae33b>] mutex_lock_nested+0x14b/0x470
>> [ 725.897860] [<ffffffff811e656f>] ? mem_hotplug_begin+0x4f/0x80
>> [ 725.916698] [<ffffffff811e656f>] mem_hotplug_begin+0x4f/0x80
>> [ 725.935064] [<ffffffff811e6525>] ? mem_hotplug_begin+0x5/0x80
>> [ 725.953464] [<ffffffff81a9631b>] online_pages+0x3b/0x520
>> [ 725.971542] [<ffffffff815eb0b3>] ? device_online+0x23/0xa0
>> [ 725.989207] [<ffffffff81601524>] memory_subsys_online+0x64/0xc0
>> [ 726.008513] [<ffffffff815eb0fd>] device_online+0x6d/0xa0
>> [ 726.025579] [<ffffffff816012eb>] store_mem_state+0x5b/0xe0
>> [ 726.043400] [<ffffffff815e8258>] dev_attr_store+0x18/0x30
>> [ 726.060506] [<ffffffff8127a808>] sysfs_kf_write+0x48/0x60
>> [ 726.077940] [<ffffffff81279d1b>] kernfs_fop_write+0x13b/0x1a0
>> [ 726.099416] [<ffffffff811f9f67>] vfs_write+0xb7/0x1f0
>> [ 726.115748] [<ffffffff811fabf8>] SyS_write+0x58/0xd0
>> [ 726.131933] [<ffffffff81ab1ea9>] system_call_fastpath+0x12/0x17
>> [ 726.150691] 7 locks held by systemd-udevd/888:
>> [ 726.165044] #0: (sb_writers#3){......}, at: [<ffffffff811fa063>] vfs_write+0x1b3/0x1f0
>> [ 726.192422] #1: (&of->mutex){......}, at: [<ffffffff81279c46>] kernfs_fop_write+0x66/0x1a0
>> [ 726.220289] #2: (s_active#60){......}, at: [<ffffffff81279c4e>] kernfs_fop_write+0x6e/0x1a0
>> [ 726.249382] #3: (device_hotplug_lock){......}, at: [<ffffffff815e9c15>] lock_device_hotplug_sysfs+0x15/0x50
>> [ 726.281901] #4: (&dev->mutex){......}, at: [<ffffffff815eb0b3>] device_online+0x23/0xa0
>> [ 726.308619] #5: (mem_hotplug.lock){......}, at: [<ffffffff811e6525>] mem_hotplug_begin+0x5/0x80
>> [ 726.337994] #6: (mem_hotplug.lock#2){......}, at: [<ffffffff811e656f>] mem_hotplug_begin+0x4f/0x80
>>
>> In short: onlining grabs device lock and then tries to do mem_hotplug_begin()
>> while add_memory() is between mem_hotplug_begin() and mem_hotplug_done() and it
>> tries grabbing device lock.
>>
>> To my understanding ACPI memory hotplug doesn't have the same issue as
>> device_hotplug_lock is being grabbed when the ACPI device is added.
>>
>> Solve the issue by grabbing device_hotplug_lock before doing add_memory(). If
>> we do that, lock_device_hotplug_sysfs() will cause syscall retry which will
>> eventually succeed. To support the change we need to export lock_device_hotplug/
>> unlock_device_hotplug. This approach can be completely wrong though.
>
> Saying the approach could be completely wrong doesn't inspire a lot of
> confidence. I assume this output is from the hung task detector, is there
> any other lockdep output that suggests there's a possible deadlock?
I said 'can be completely wrong' not because I'm not sure about the
cause of the deadlock (see locks #.2,3,5 in worker thread and locks
4,5,6 in systemd-udev) and not because I'm not sure my patch solves the
issue (as you can see lock #3 in systemd-udevd was taken before the
dev->mutex so we should be safe). My testing also showed the issue is
gone. I rather wasn't sure there is no other way to obtain this lock
indirectly or do some other synchronization.
--
Vitaly
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